An organic light-emitting diode device, also called an OLED device, commonly includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
There is a continuing need for efficient, stable, robust formulations for broadband light-emitting OLED devices. By broadband light, it is meant that the OLED emits sufficient light throughout the spectrum so that such light can be used in conjunction with filters to produce a full color display. In particular, there is a need for white light-emitting OLEDs where there is substantial emission in the red, green, and blue portions of the spectrum, wherein a white emitting electroluminescent (EL) layer can be used to form a multicolor device. Each pixel is coupled with a color filter element as part of a color filter array (CFA) to achieve a pixilated multicolor display. The organic EL layer is common to all pixels and the final color as perceived by the viewer is dictated by that pixel's corresponding color filter element. Therefore, a multicolor or RGB device can be produced without requiring any patterning of the organic EL layers. An example of a white CFA top emitting device is shown in U.S. Pat. No. 6,392,340.
White light producing OLED devices should be bright, efficient, and generally have 1931 Commission Internationale d'Eclairage (CIE) chromaticity coordinates, (CIE x, CIE y), of about (0.33, 0.33). The following patents and publications disclose the preparation of OLED devices capable of producing white light, comprising a hole-transporting layer and an organic luminescent layer, and interposed between a pair of electrodes.
White light producing OLED devices have been reported by J. Shi (U.S. Pat. No. 5,683,823) wherein the luminescent layer includes red and blue light-emitting materials uniformly dispersed in a host emitting material. Sato et al. in JP 07-142169 discloses an OLED device, capable of emitting white light, made by forming a blue light-emitting layer next to the hole-transporting layer and followed by a green light-emitting layer having a region containing a red fluorescent layer.
Kido et al., in Science, 267, 1332 (1995) and in Applied Physics Letters, 64, 815 (1994), report a white light-producing OLED device. In this device, three emitter layers with different carrier transport properties, each emitting blue, green, or red light, are used to produce white light. Littman et al. in U.S. Pat. No. 5,405,709 disclose another white emitting device, which is capable of emitting white light in response to hole-electron recombination, and comprises a fluorescent in a visible light range from bluish green to red. More recently, Deshpande et al., in Applied Physics Letters, 75, 888 (1999), published a white OLED device using red, blue, and green luminescent layers separated by a hole-blocking layer.
As illustrated above, a common approach to the construction of a white light producing OLED device is to combine layers with different emission spectra. A frequent problem with such combinations is that the relative intensities of emission of the different layers may vary with the current density, resulting in an overall emission spectrum and emission color that also vary with current density. Such current dependences require complicated algorithms to adjust the intensities of differently colored pixels to achieve a desired overall emission color.
Kobori et al., in Unexamined Patent Application JP 2001-52870, teach the use of a host comprising a mixture of an anthracene derivative and an aromatic amine for a blue light-emitting layer and, if present, other light-emitting layers. They disclose a white light-emitting OLED having two light-emitting layers constructed in this manner. In the examples disclosed, both light-emitting layers include a mixture of an aromatic amine and a bisanthracene compound in a 25%/75% ratio as a host. The first light-emitting layer (which is closer to the anode) includes a rubrene derivative as a yellow light-emitting material doped into the host in a few percent. To make white light, a second light-emitting layer (closer to the cathode) is provided on the first light-emitting layer. The second light-emitting layer uses an arylamine-substituted styrene derivative as the blue light-emitting compound doped into the host. In the example, an electron-transporting layer is provided over the second light-emitting layer, an alkali metal halide electron-injecting layer (CsI) disposed over the electron-transporting layer, and a Mg:Ag alloy cathode deposited over the CsI.
Although the OLED disclosed in JP 2001-52870 provides an adequate white color with effective lifetime, it is not a robust formulation. For example, merely removing the alkali metal halide electron-injecting layer results in a dramatic shift to yellow emission, with 90% or more of the emission coming from the first light-emitting layer. This also resulted in a significant decrease in efficiency and lifetime. Further, it was indicated that the decrease in lifetime was especially large for the blue. It is known in the art that an Mg:Ag cathode provides effective performance without an alkali metal halide layer. Such a dramatic shift in performance based solely on the presence or absence of an alkali metal halide layer is unacceptable from a manufacturing perspective. This indicates that the color, efficiency, and lifetime of this structure are very sensitive to the electron-injecting properties of the OLED. In manufacturing, an OLED formulation should be robust to variables that can arise in the manufacturing process. Some of these variables relate to manufacturing tolerances and can include chemical composition variations, thickness variations, variations in electron- and hole-injecting properties, and so forth. Some other variables relate to degrees of freedom in selection of processes and materials, including the cathode. For various reasons (reflectivity, conductivity, ease of manufacture) may need to change the cathode, but without reformulating the device.
U.S. Pat. No. 5,885,498 discloses the use of symmetric anthracene compounds such as 9,10-diphenylanthracene (DPA) doped into a high Tg (glass transition temperature) hole-transporting layer at low concentrations, wherein the DPA emits light. It is alleged that such a device has higher efficiency and a reduced number of black spots that form upon keeping. It does not disclose that such a mixture is useful as a host material.